We Irish are used to invasions. Vikings, Normans, the British we've seen them all come and go – or come and stay.

Now we've got another one on our hands, by the exotic name of Chalara fraxinea. This invader isn't as obvious as the others, but is just as deadly.

Chalara fraxinea is a fungal spore that attacks ash trees. It has infected 90pc of the ash trees in Denmark, and thousands in Germany, Poland and swathes of Eastern Europe.

It has been identified in 11 sites in Ireland – the result of an infected batch of 30,000 trees imported from Holland three years ago, according to Minister of State Shane McEntee – and all the plantations have been destroyed. Now, we're into a waiting game to see if it has had time to spread.

Because once it does, it is invariably fatal. When the Danes first noticed it in 2003, they thought the split tree bark and black leaves were the result of frost. By 2005, the full scale of the disease was apparent: there was no way of stopping it. Then they discovered that it had been killing Polish ash trees since 1992.

"This is a very serious disease," says Mary Keenan, director of the Tree Council of Ireland. "We have planted a lot of ash trees in the last 10 to 15 years as we moved more towards broadleaf deciduous trees.

"It seems to be even more aggressive than Dutch elm disease, which virtually wiped out the elm species in Ireland."

Scientists think it came originally from Asia. It spread by air from Poland into Norway, Sweden and Denmark. It has been found in 52 locations in the UK.

The Norwegians tried burning infected copses – as we have done in Leitrim, Galway, Meath and Monaghan – to no effect.

"We can't see any point burning the trees, and you can't burn the air," Ditte Olrik, a biologist with the Danish Nature Agency, said.

Chalara fraxinea is the latest in a long line of unwanted visitors, both vegetable and animal, to make it on to our island.

John Kelly of Invasive Species Ireland – a joint venture between the Republic's National Parks and Wildlife Service and Northern Ireland's Department of the Environment – spends his time monitoring unwanted invaders.

"We collect data and records and pass on information to the relevant agencies," he says, "but sometimes there is little you can do."

John has recorded all sorts of exotic escapees, from wild boar to snapper turtles. However, his public enemy number one is the rhododendron.

"It is very invasive, and grows in dense thickets," he says, "but not only that, it harbours Phytophthora ramorum, also known as 'sudden oak death'."

Foreign invaders are affecting ecosystems all over the country. And once established, they are very difficult to eradicate, as they often outbreed native species.

Here, then, is our top 10 of unwanted visitors:

1 The Asian Clam: "It's been found in the Barrow river system and in Lough Derg," says John Kelly of Invasive Species Ireland. "It just out-competes all native species and affects fish breeding grounds."

2Curly waterweed: this green water plant from Africa is aggressively invasive, and can take over any body of water up to six metres deep. It affects fish and insect life. It was introduced as an ornamental aquarium plant.

3 Wild ferret: introduced to keep rabbit numbers down, this non-native version of the wild polecat is found in several Irish counties and preys on birds and affects the breeding habits of rare bird species.

4 Giant hogweed: this member of the carrot family sprouts giant leaves that shade and deplete its rivals. It produces a toxic sap that causes human skin to blister.

5 Grey squirrel: this American species was introduced as a novelty on Irish landed estates. It has all but done for our native Red Squirrel: it's bigger, stronger and carries the Parapox disease that kills Red Squirrels.

6 The Hottentot Fig: this South African plant was introduced to Ireland as an ornamental species. It has now spread to dunelands on the east coast and is affecting pH levels and nutrient recycling.

7 Japanese knotweed: hated by gardeners everywhere, this Asian species is almost impossible to eradicate. It can even grow through concrete. It affects biodiversity and occludes native species.

8New Zealand flatworm: Introduced into Northern Ireland in the 1960s, the species is now found throughout Ireland. It reduces the number of native earthworms, which in turn has a huge impact on soil aeration and fertility.

9The Zebra Mussel: this invader, originally from the Caspian Sea, is now found in the Shannon, Boyle and Erne systems and in Lough Neagh. It attaches itself to native mussels, killing them in the process.

10The Harlequin Ladybird: The Harlequin eats ladybirds and takes their food as well. Originally from Asia, this ladybird can wipe out native species.

For more information, or to report a sighting, visit www.invasivespeciesireland.com